As used herein, the term computer includes any device or machine capable of accepting data, applying prescribed processes to the data, and supplying the results of the processes. A multiprocessing computer system has multiple processes executing on the system. Each process performs a particular task, and the processes, taken as a whole, perform some larger task, typically called an application. These processes may be executing on a single central computer or they may be running on separate computers which are connected to each other via some type of communications link, i.e., a distributed or networked computer system.
In multiprocessing systems, resources are often shared among the executing processes. Such resources may include, for example, disk drives, printers, shared memory and databases. During processing, a process may require exclusive access to a resource, such that another process may not use that resource until the first process is finished with it. Thus, several processes may compete for a finite number of resources. This is commonly known as mutually exclusive sharing of resources.
A problem with mutually exclusive access to resources is the possibility that the computer system will enter a deadlock state. A deadlock state is a state in the computer system in which, because of a resource allocation pattern, the computer system cannot progress past a processing point. For example, consider a computer system with two resources R1 and R2, and two processors P1 and P2, where P1 and P2 both need simultaneous exclusive access to R1 and R2 at some point in order to successfully complete their processing. If P1 gains exclusive access to R1 and R2, then P2 must wait until P1 releases the resources. In this situation, P2 is described as being in a “wait” state. Such a situation does not present a problem, because, it is presumed, that P1 will eventually release the resources, at which time P2 may gain access to the resources. However, consider the situation in which P1 gains access to and holds R1, and P2 gains access to and holds R2, as shown in FIG. 1. If this occurs, then P1 cannot finish its task until it gains access to R2, and P2 cannot finish its task until it gains access to R1, i.e., both P1 and P2 will enter a wait state. However, P1 is holding R1 and will not release R1 until it gains access to R2, and P2 is holding R2 and will not release R2 until it gains access to R1. At this point, the system is in a deadlock state.
Systems and mechanisms for deadlock avoidance and recovery are known in the art and described in U.S. Pat. No. 5,664,088 to Romanovsky et al. and U.S. Pat. No. 5,913,060 to Discavage.
Generally, in the prior art, once a deadlock is detected, one of the processes involved in the deadlock is terminated, so that it releases the resource it held, and the resource can be reclaimed by the system. The reclaimed resource may then be used by a waiting process. If the waiting process can finish processing using the reclaimed resource, then the system can progress past the deadlock state. The terminated process, called the victim, is generally selected on a random basis, or based on a static priority assigned to the processes.
In addition to the “circular” deadlock situation shown in FIG. 1, a deadlock may also be caused by processors failing, or “crashing,” before they release a resource. For example, in the same two processor system described above, assume processor P1 holds resource R1, and processor P2 holds resource R2 and is waiting for R1, as shown in FIG. 2. If processor P1 then unexpectedly crashes while still holding resource R1, P2 remains in a wait state, aware that R1 is busy, but not aware that P1 has crashed. Thus, P2 will wait forever for R1, which is being held by crashed processor P1, causing a deadlock state. Since prior art deadlock avoidance schemes are based on the assumption that processors will never malfunction, these schemes will not prevent deadlocks caused by crashing processors.
In any distributed or parallel computer system, access to shared resources is controlled by some form of a locking mechanism or scheme, whereby a shared resource is committed to, or “locked” by, a single holding processor until that processor releases the resource (i.e., releases the “lock”). To make such a system fault-tolerant, when a lock holding processor (i.e., a processor that has locked a resource) unexpectedly crashes, a rescuing method is needed to prevent system deadlock. The rescue operation will inherently be determined by the locking scheme. Problems may arise when two or more processors try to rescue the same locked resource. The correct behavior is that exactly one of them would succeed. Thus, there is a need for a generic fault-tolerant locking mechanism or scheme that can avoid deadlocks caused by failing processors in a multiprocessor system, especially mission-critical parallel systems where high availability is absolutely necessary. Such a mechanism will enable waiting processors to identify a locked resource held by a failed processor, and “rescue” the resource from the hold of its failed processor, by changing, or “re-setting,” a lock associated with the resource. A single waiting processor will then rescue a resource to prevent the potential deadlock from locking up all other processors waiting for the same resource.